1. Field of the Invention
The invention relates to a process for determining wear on the clutch disk of a friction clutch, which is pressed by a pressure plate arranged in the clutch housing, against a flywheel of an internal combustion engine. The invention further relates to a device with which this process can be implemented.
2. Discussion of the Prior Art
In light of the longer maintenance intervals sought by automobile manufacturers, it is desirable to have a device for determining wear on a clutch lining, so that the clutch disk can be exchanged in a timely manner, without any visual examination being needed in the meantime. It is conceivable either to continually determine, check and display the friction lining wear, or else to determine only a maximum permissible wear limit that, when reached, is displayed via a signal device.
For clutches in commercial vehicles, it is helpful to know the state of clutch wear, because unforeseen downtime results in costs that could have been avoided with better planning. Unpredictable wear-related factors can arise when the clutch is subjected to unusual stress due to other or different drivers or temporarily harder conditions of use.
German reference DE 26 40 088 A1 discloses a device for determining wear on a clutch disk lining, in which the axial position of the pressure plate is detected by a sensor. There is a marking on the pressure plate, which marking consists of two marks that extend in the axial direction of the clutch and are located one after the other in the circumferential direction of the pressure plate and of a third mark that connects the front end of one of the two marks to the rear end of the other mark (i.e., a Z-shape). An inductive sensor is inserted in an opening of the clutch housing. The sensor and the marking are oriented to each other in such a way that when the clutch is closed and the clutch disk lining is new, the effective direction of the sensor overlays a center line of the Z-shaped marking. Connected to the sensor is an analysis unit that has a display unit. The marking is located on the edge of the pressure plate so that, when the clutch is rotated, the marking passes in front of the sensor at a uniform distance. The sensor detects three passages. When the friction lining is new, the passage signals are equidistant from each other. When the friction lining becomes worn, the pressure plate, and with it the Z-shaped marking, moves farther in the direction of the flywheel. As a result, the effective direction of the sensor deviates from the center line, and a different chronological passage sequence is detected. From the chronological signal sequence, it is possible, via the electronic analysis unit, to determine the axial movement distance of the pressure plate and thus the wear of the friction lining, which can then be displayed.
Because the marking and the mechanical device to which the marking is attached are subject to mechanical play as well as to the influences of temperature and speed, additional axial movements of the marking occur. These cannot be distinguished from movements that result from wear. In this event, the signals are erroneous and indicate either too much wear or too little wear. In the former case, the clutch will be overhauled prematurely (which may increase maintenance costs only insignificantly). However, in the latter case, the danger exists that the clutch disk will be overhauled too late and that damage will be suffered by the flywheel as the rivets securing the friction linings to the clutch disk work themselves into the flywheel.
German reference DE 29 16 807 A1 discloses a distance sensing device for a friction clutch, in which pins that rest on a compression spring, are let into in the flywheel. The movement of a pin is transmitted via a lever to a ring, which is movable in the axial direction and interacts with a stationary approach switch arranged on the clutch housing. When the clutch is closed, the pins are pressed by the clutch disk against the force of the compression spring into the flywheel. This distance sensing device is intended to determine, particularly in trucks, the exact operational point at which the clutch closes, so as to reduce the operational times of the pneumatic clutch, which can be operated quickly at first and then, after the contact for engagement, can again be operated slowly. With these sensors, wear on the friction lining of the clutch disk upon engagement can be taken into account, but not determined.